Oh dear, I'm coming up on two and a half years on HN (time flies!) and I didn't know either...
Don't worry, it's recent and mostly just designed to help you figure out if you should flag an article as a dupe.
Never seen that button. Speaks for the UI designer, that a comment about a function is more visible than the function itself ;-) Just kidding, I am a fault for not looking right!
Whilst we're on the subject, there's another function you may have missed. If you've got showdead on (via your account settings) and you see a dead post or story that you thought was worth sharing, click on the timestamp by the post and click on the 'vouch' link there to suggest that it shouldn't be hidden.